I've got two computers both running Firebird (let's called them A and B) and two database files (let's call them a.fdb and b.fdb), both copied to both computers. On B I get the message
b.fdb is not a valid database The other files work, on A both files work.
This would be easy to explain, if B was an older version, but it's a newer one (it was FB 2.1, and because of this problem I installed FB 2.5). The files a.fdb and b.fdb have ODS 11.1 and 10.1, respectively. It looks like new versions of FB wouldn't support ODS 10.1 anymore, which I can't believe. But I see no other explanation, can you?
Try to make backup and restore of database.